In the related art, in a space such as stations, airports, and the like used by a large number of unspecified persons, monitoring of unattended suspicious objects has been performed for the purpose of crime prevention and the like. Monitoring of an unattended object is generally performed by an observer monitoring captured images of a monitoring area imaged by a monitoring camera (imaging device), but it is necessary to automatically detect an unattended object based on captured images of a monitoring area in order to reduce the burden on the observer and improve the accuracy of the detection of the unattended object. Therefore, various techniques for automatically detecting an unattended object based on captured images have been proposed (see PTL 1). In the technique in the related art of PTL 1, a difference between an input image of a monitoring target area captured and input at a predetermined time interval and an initial image of the monitoring target area is obtained to cut out the image of a person in the monitoring target area, the person images successively obtained at the predetermined time interval are compared with each other to detect an object image to be separated from the person images, and when the object image is detected, the time for which the object image is left behind measured so that an alarm is issued when a predetermined time has elapsed.
In addition, in a case where an unattended object is detected, it is desirable to be able to specify the owner of the unattended object. Therefore, various techniques have been proposed for specifying the owner of an unattended object based on captured images (see PTL 2 and PTL 3). In the technique in the related art of PTL 2, an image of an unattended object and an image of a surrounding person within a predetermined distance range from the unattended object are extracted from a captured image, and log data including the image of the unattended object and the image of the surrounding person are extracted so as to provide a user with information for specifying the owner of the unattended object. In addition, in the technique in the related art of PTL 3, in a predetermined period with an end point just before the detection time at which an unattended object (suspicious object) is first detected, all the objects whose detection areas overlap with a detection area of the suspicious object are detected as a candidate object of the owner of the unattended object, and information (incident information) including an object image of the detected candidate object is output so as to provide the user with information for specifying the owner of the unattended object.